Limits...
Vision and foraging in cormorants: more like herons than hawks?

White CR, Day N, Butler PJ, Martin GR - PLoS ONE (2007)

Bottom Line: We measured the aquatic visual acuity of great cormorants under a range of viewing conditions (illuminance, target contrast, viewing distance) and found it to be unexpectedly poor.We conclude that cormorants are not the aquatic equivalent of hawks.This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons.

View Article: PubMed Central - PubMed

Affiliation: Centre for Ornithology, School of Biosciences, University of Birmingham, Birmingham, United Kingdom.

ABSTRACT

Background: Great cormorants (Phalacrocorax carbo L.) show the highest known foraging yield for a marine predator and they are often perceived to be in conflict with human economic interests. They are generally regarded as visually-guided, pursuit-dive foragers, so it would be expected that cormorants have excellent vision much like aerial predators, such as hawks which detect and pursue prey from a distance. Indeed cormorant eyes appear to show some specific adaptations to the amphibious life style. They are reported to have a highly pliable lens and powerful intraocular muscles which are thought to accommodate for the loss of corneal refractive power that accompanies immersion and ensures a well focussed image on the retina. However, nothing is known of the visual performance of these birds and how this might influence their prey capture technique.

Methodology/principal findings: We measured the aquatic visual acuity of great cormorants under a range of viewing conditions (illuminance, target contrast, viewing distance) and found it to be unexpectedly poor. Cormorant visual acuity under a range of viewing conditions is in fact comparable to unaided humans under water, and very inferior to that of aerial predators. We present a prey detectability model based upon the known acuity of cormorants at different illuminances, target contrasts and viewing distances. This shows that cormorants are able to detect individual prey only at close range (less than 1 m).

Conclusions/significance: We conclude that cormorants are not the aquatic equivalent of hawks. Their efficient hunting involves the use of specialised foraging techniques which employ brief short-distance pursuit and/or rapid neck extension to capture prey that is visually detected or flushed only at short range. This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons.

Show MeSH

Related in: MedlinePlus

Prey detectability model for a great cormorant demonstrating the effect of illumination.The model is based upon a great cormorant foraging on a capelin (Mallotus villosus, 10 cm TL) of 60% contrast viewed at a distance of 1.05 m over a range of ambient illumination levels equivalent to those received at the earth's surface from natural sources between daylight to a moonless night. These span the range of target light levels used in this series of experiments and span the range of ambient light levels that are known to be encountered by cormorants during natural dives [18] (ca 0.5 to 100 lux). Each frame depicts a scene with an angular width of 10°.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC1919429&req=5

pone-0000639-g006: Prey detectability model for a great cormorant demonstrating the effect of illumination.The model is based upon a great cormorant foraging on a capelin (Mallotus villosus, 10 cm TL) of 60% contrast viewed at a distance of 1.05 m over a range of ambient illumination levels equivalent to those received at the earth's surface from natural sources between daylight to a moonless night. These span the range of target light levels used in this series of experiments and span the range of ambient light levels that are known to be encountered by cormorants during natural dives [18] (ca 0.5 to 100 lux). Each frame depicts a scene with an angular width of 10°.

Mentions: To explore the consequences of the cormorants' poor visual resolution we have used our acuity data to model prey detectability under a range of viewing conditions. We used curves fitted to our acuity-illumination (Fig. 1), acuity-contrast (Fig. 2), and acuity-viewing distance (Fig. 3) functions to describe a series of “threshold acuity surfaces” which relate acuity to target contrast and illumination, for each viewing distance (Fig. 4). This encapsulates within a single figure the ways in which acuity is influenced by a range of important parameters that describe the visual tasks encountered by foraging cormorants. From this we have been able to model visual prey detectability in cormorants under a range of viewing conditions. Figs 5 and 6 show two examples from this modelling using a prey item of a size (10 cm total length) commonly taken by cormorants [24], [25]. Even for a prey item of this size detectability is low at all but the highest target contrasts, light levels and short viewing distances. This raises a number of important questions concerning the foraging techniques of cormorants and the predator-prey interactions which underlie them.


Vision and foraging in cormorants: more like herons than hawks?

White CR, Day N, Butler PJ, Martin GR - PLoS ONE (2007)

Prey detectability model for a great cormorant demonstrating the effect of illumination.The model is based upon a great cormorant foraging on a capelin (Mallotus villosus, 10 cm TL) of 60% contrast viewed at a distance of 1.05 m over a range of ambient illumination levels equivalent to those received at the earth's surface from natural sources between daylight to a moonless night. These span the range of target light levels used in this series of experiments and span the range of ambient light levels that are known to be encountered by cormorants during natural dives [18] (ca 0.5 to 100 lux). Each frame depicts a scene with an angular width of 10°.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC1919429&req=5

pone-0000639-g006: Prey detectability model for a great cormorant demonstrating the effect of illumination.The model is based upon a great cormorant foraging on a capelin (Mallotus villosus, 10 cm TL) of 60% contrast viewed at a distance of 1.05 m over a range of ambient illumination levels equivalent to those received at the earth's surface from natural sources between daylight to a moonless night. These span the range of target light levels used in this series of experiments and span the range of ambient light levels that are known to be encountered by cormorants during natural dives [18] (ca 0.5 to 100 lux). Each frame depicts a scene with an angular width of 10°.
Mentions: To explore the consequences of the cormorants' poor visual resolution we have used our acuity data to model prey detectability under a range of viewing conditions. We used curves fitted to our acuity-illumination (Fig. 1), acuity-contrast (Fig. 2), and acuity-viewing distance (Fig. 3) functions to describe a series of “threshold acuity surfaces” which relate acuity to target contrast and illumination, for each viewing distance (Fig. 4). This encapsulates within a single figure the ways in which acuity is influenced by a range of important parameters that describe the visual tasks encountered by foraging cormorants. From this we have been able to model visual prey detectability in cormorants under a range of viewing conditions. Figs 5 and 6 show two examples from this modelling using a prey item of a size (10 cm total length) commonly taken by cormorants [24], [25]. Even for a prey item of this size detectability is low at all but the highest target contrasts, light levels and short viewing distances. This raises a number of important questions concerning the foraging techniques of cormorants and the predator-prey interactions which underlie them.

Bottom Line: We measured the aquatic visual acuity of great cormorants under a range of viewing conditions (illuminance, target contrast, viewing distance) and found it to be unexpectedly poor.We conclude that cormorants are not the aquatic equivalent of hawks.This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons.

View Article: PubMed Central - PubMed

Affiliation: Centre for Ornithology, School of Biosciences, University of Birmingham, Birmingham, United Kingdom.

ABSTRACT

Background: Great cormorants (Phalacrocorax carbo L.) show the highest known foraging yield for a marine predator and they are often perceived to be in conflict with human economic interests. They are generally regarded as visually-guided, pursuit-dive foragers, so it would be expected that cormorants have excellent vision much like aerial predators, such as hawks which detect and pursue prey from a distance. Indeed cormorant eyes appear to show some specific adaptations to the amphibious life style. They are reported to have a highly pliable lens and powerful intraocular muscles which are thought to accommodate for the loss of corneal refractive power that accompanies immersion and ensures a well focussed image on the retina. However, nothing is known of the visual performance of these birds and how this might influence their prey capture technique.

Methodology/principal findings: We measured the aquatic visual acuity of great cormorants under a range of viewing conditions (illuminance, target contrast, viewing distance) and found it to be unexpectedly poor. Cormorant visual acuity under a range of viewing conditions is in fact comparable to unaided humans under water, and very inferior to that of aerial predators. We present a prey detectability model based upon the known acuity of cormorants at different illuminances, target contrasts and viewing distances. This shows that cormorants are able to detect individual prey only at close range (less than 1 m).

Conclusions/significance: We conclude that cormorants are not the aquatic equivalent of hawks. Their efficient hunting involves the use of specialised foraging techniques which employ brief short-distance pursuit and/or rapid neck extension to capture prey that is visually detected or flushed only at short range. This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons.

Show MeSH
Related in: MedlinePlus